As means for detecting an NMR signal, an NMR probe head including an NMR antenna coil has been known. The NMR antenna coil, which is typically cylindrical, irradiates a sample inserted therein with a RF (radio frequency) pulse, and picks up a magnetic resonance signal radiated from the sample when a predetermined period of time has elapsed since the irradiation. The NMR signal thusly detected is very weak, and hence it is necessary to increase detection sensitivity.
A technique has been developed as a method for increasing the detection sensitivity, the technique which reduces thermal noise by cooling the antenna coil with a refrigerant such as helium to a cryogenic temperature. Referring to Expression (1) given below, the S/N ratio (SNR) of an NMR signal is inversely proportional to the square root of a value including the product of a temperature Tc and a resistance Rc of the antenna coil. The cooling the antenna coil markedly increases the detection sensitivity of an NMR signal.SNR∝1/√[TcRc+TsRs+Ta(Rc+Rs)2]  (1)where Ts is a temperature of a sample to be a measurement subject, Rs is an effective resistance of the sample, and Ta is a temperature of a preamplifier that amplifies a signal.
Patent Document 1 discloses, as means for cooling the antenna coil, a probe head including a cylindrical thermal conductor that supports the antenna coil, and a heat exchanger that is fed by helium gas cooled by a GM refrigerator. More specifically, FIG. 2 in Patent Document 1 shows a structure in which a lower end of the thermal conductor is coupled to an upper surface of the heat exchanger in an upstanding state.
Obviously from aforementioned Expression (1), the temperature of the antenna coil is preferably as cold as possible to increase the S/N ratio of an NMR signal. However, with the conventional cooled NMR probe head disclosed in Patent Document 1, the antenna coil is operated at temperatures in a range of from 20K to 25K. Such temperatures define the cooling limit. The reason is given below.
As described above, the NMR antenna coil typically serves as both NMR detection and RF transmission coils. Thus, the NMR antenna coil receives electrical power for measurement. The electrical power is partly consumed as Joule heat in the antenna coil; increasing the temperature of the coil. Meanwhile, the specific heat of metal (for example, copper or aluminum), which forms the antenna coil, is significantly small in a temperature range below the range of from 20K to 25K. Hence, even very small Joule heat significantly increases the temperature of the antenna coil. In addition, the electrical resistance of copper or aluminum, which is the material of the antenna coil, increases as the temperature of the material increases. The increase in the electrical resistance promotes an increase in temperature of the antenna coil due to the Joule heat. The electrical resistance may be suppressed to a very small resistance, called remnant resistance, in a low temperature range around a range of from 10K to 20K. In a higher temperature range than that temperature range, for example, in the operation temperature range (from 20K to 25K) of the conventional antenna coil, the electrical resistance rapidly increases as the temperature increases.
Patent Document 1: Specification and Drawings of US Patent Application Publication No. 2004/0004478